Not applicable.
Not applicable.
(1) Field of the Invention
The present invention generally relates to solder compositions of the type used with electronic packaging, such as flip chip packaging. More particularly, this invention relates to lead-based solder alloys requiring reflow temperatures that are compatible with typical circuit board assembly processes and exhibit improved reliability over eutectic 63Sn-37Pb and Sn-36Pb-2Ag solder alloys under high temperature processing and operating conditions.
(2) Description of the Related Art
Surface-mount (SM) semiconductor devices such as flip chips and ball grid arrays (BGA""s) are attached to circuit boards with beadlike terminals formed on interconnect pads located on one surface of the device. The terminals are typically in the form of solder bumps near the edges of the chip, which are reflowed to both secure the chip to the circuit board and electrically interconnect the flip chip circuitry to a conductor pattern on the circuit board. Reflow soldering techniques typically entail depositing a controlled quantity of solder on the pads of the chip using methods such as electrodeposition and printing, and then heating the solder above its melting or liquidus temperature (for eutectic and noneutectic alloys, respectively) to form the solder bumps on the pads. After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reheating, or reflowing, the solder so as to form solder connections that metallurgically adhere to the conductors. The maximum temperature achieved during the reflow process is referred to as the peak reflow temperature; which is conventionally about 20xc2x0 C. to about 50xc2x0 C. above the melting or liquidus temperature of the particular solder alloy.
Flip chip interconnect pads are electrically interconnected with the circuitry on the flip chip through vias. Because aluminum metallization is typically used in the fabrication of integrated circuits, interconnect pads are typically aluminum or aluminum alloy, which are generally unsolderable and susceptible to corrosion if left exposed. Consequently, one or more additional metal layers are often deposited on the pads to promote wetting and metallurgical bonding with solder bump alloys. These additional metal layers, referred to as under bump metallurgy (UBM), may be, for example, sputtered nickel and copper, respectively, or an evaporated multilayer structure of chromium, a diffusion barrier layer of a chromium-copper alloy, and a solderable layer of copper. In each example, copper forms the outer layer of the UBM because it is readily solderable, i.e., can be wetted by and will metallurgically bond with solder alloys of the type used for solder bumps.
FIG. 1 represents a cross-section through a solder bump connection or joint 12 of a flip chip 10 attached to a circuit board 14, such as an organic circuit board known in the industry as FR-4, though the chip 10 could be mounted to a flexible circuit, ceramic or silicon substrate, or another suitable material. The solder joint 12 is bonded to an aluminum runner 16 on the chip 10 and a copper trace 18 on the board 14, thereby electrically and mechanically connecting the chip 10 to the board 14. As shown, a portion of the runner 16 is exposed by an opening in a passivation layer 22 to define an interconnect pad on which a UBM 20 has been deposited. The solder joint 12 has a spherical shape characteristic of a reflowed solder bump alloy, such as the eutectic 63Sn-37Pb solder alloy (melting point of 183.0xc2x0 C.) and the eutectic 62Sn-36Pb-2Ag solder alloy (melting point of 178.8xc2x0 C.) used for flip chip assemblies. The peak reflow temperature for the eutectic Sn-37Pb and Sn-36Pb-2Ag alloys is about 205xc2x0 C. to about 240xc2x0 C. Various other alloys are commercially available and used for flip chip assembly, including Pb-free alloys such as Sn-5Ag, eutectic Sn-3.5Ag and eutectic Sn-0.9Cu, and their derivatives including Sn-4.0Ag-0.5Cu, Sn-3.9Ag-0.6Cu, Sn-3.8Ag-0.7Cu, Sn-4Ag-1Cu and Sn-4.7Ag-1.7Cu. To be compatible with widely-used FR-4 circuit board assembly processes, the maximum reflow temperature of a solder alloy must not be higher than about 260xc2x0 C., and preferably not higher than 250xc2x0 C., in order to avoid damage to the circuit board and its components through board warping, pop-corning, delamination, etc. In view of these considerations, eutectic Sn-37Pb and Sn-36Pb-2Ag alloys have found wide use for reflow assembly processes, while eutectic Snxe2x80x94Agxe2x80x94Cu and near-eutectic Snxe2x80x94Agxe2x80x94Cu alloys have found use where lead-free solders are required.
In addition to having acceptable reflow characteristics, another consideration when selecting solder alloys for flip chip applications is reliability. In solder joints formed of lead-free SnAg and SnAgCu alloys, electromigration of UBM material in the direction of electron flux has been identified as leading to excessive resistances and open solder connections. When a thin-film UBM is used, such as a sputtered Alxe2x80x94NiVxe2x80x94Cu metallization or evaporated Crxe2x80x94CrCuxe2x80x94Cuxe2x80x94Au metallization, this mass transport mechanism can become the predominant failure mechanism under severe service conditions, such as high temperature and/or current applications. In copending and co-assigned U.S. patent application Ser. No. 10/099,861, a noneutectic lead-free solder alloy containing tin, silver and copper is disclosed that inhibits UBM electromigration, and is therefore capable of exhibiting improved reliability over lead-free eutectic Snxe2x80x94Agxe2x80x94Cu solders, while maintaining a peak reflow temperature near that of the eutectic SnAg and eutectic SnAgCu alloys.
Lead-based solder alloys used in high temperature and high power applications also exhibit excessive resistances and open connections associated with several different diffusion-related mechanisms. It would be desirable if the reliability of lead-based solder alloys could be improved.
The present invention is directed to a tin-lead solder alloy containing copper and optionally silver, nickel, palladium, platinum and/or gold as its alloying constituents. The solder alloy consists essentially of, by weight, about 55% to about 75% tin, about 11% to about 44% lead, up to about 4% silver, nickel, palladium, platinum and/or gold, greater than 0.5% (preferably greater than 1%) to about 10% copper, and incidental impurities. Because of the similar physical and chemical properties that copper and nickel exhibit in soldering applications, it is believed that nickel can be substituted in equal weight amounts for some or all of the copper in the solder alloy. An example of a SnPbCu alloy consists essentially of, by weight, about 60% to about 62% tin, about 34% to about 36% lead, about 2% to about 4% copper, and incidental impurities. An example of a SnPbAgCu alloy consists essentially of, by weight, about 56% to about 61% tin, about 32% to about 35% lead, about 2% to about 4% silver, about 2% to about 10% copper, and incidental impurities.
Certain solder alloys of this invention appear to be eutectic and therefore characterized by a true melting temperature, while others are noneutectic and therefore characterized by distinct solidus and liquidus temperatures. The noneutectic alloys have a solidus temperature near the melting temperatures of the eutectic alloys, and may have a liquidus temperature of up to about 470xc2x0 C. However, the melting mechanism exhibited by the noneutectic alloys is such that they are substantially all melted and do not exhibit a xe2x80x9cmushy zonexe2x80x9d within a narrow temperature range, and therefore are said to have an xe2x80x9ceffective melting temperaturexe2x80x9d in which the alloys behave similarly to the eutectic alloys even though their actual liquidus temperatures are considerably higher. As a result, alloys of this invention containing up to 10 weight percent copper reflow at temperatures much lower than their actual liquidus temperatures, to the extent that these compositions can be treated as requiring peak reflow temperatures of about 250xc2x0 C. or less.
In addition to their favorable reflow characteristics, copper is present in the solder alloys of this invention in amounts sufficient to precipitate out as CuSn intermetallic compounds (IMC""s), which have been found to deposit at the interface between the solder and a UBM on which the solder is reflowed for flip chip and BGA applications. The IMC deposit advantageously increases the effective thickness of the UBM, thereby increasing the time required for a resistive or open solder connection to occur.
Other objects and advantages of this invention will be better appreciated from the following detailed description.